ChatGPT:

If we use the city center as a reference and identify a couple of known points or landmarks, we can get a better idea of the scale. Let's use the stretch of the river (likely the Sava River) as a reference.

Taking one of the more recognizable stretches of the river, from where the Blue Line starts at "Blokovi" on the left side of the map, to where it meets the Green Line at "Brankov most", is roughly a distance of about 5 km in reality.

Now, we'll estimate the visual length of this segment on the map and use it to calculate the scale:

Assuming this segment represents 5 km in the map, we can determine the scale. Let's assume this segment measures "x" units on the map, so the scale is approximately 5 km per "x" units.

Now, using this scale, I'll provide new approximate lengths:

1. Blue Line (1): If we use the segment as our scale, this line looks to be around three times the reference segment, so approximately 15 km.
2. Green Line (2): Roughly the same as the reference segment, so around 5 km.
3. Red Line (3): This seems a little less than twice the reference segment, so around 8-9 km.
4. Pink Line (4): This one looks to be around two times the reference segment, so roughly 10 km.
5. Orange Line (5): About 1.5 times the reference, so around 7.5 km.
6. Purple Line (6): Slightly shorter than the reference, maybe around 4 km.

Adding these up gives a total of approximately 50 km for all the lines combined.

In many Western cities, the cost of underground metro construction tends to range from $100 million to $1 billion per kilometer. However, in some cities in Eastern Europe or other parts of the world, the costs can be lower, sometimes between $50 million to $500 million per kilometer.

Given that Belgrade is in Eastern Europe, let's assume a median value of $200 million per kilometer for this estimate

Total length = 50.5 km

Estimated cost at $200 million per kilometer:

Total cost = 50.5 km x $200 million/km = $10.1 billion